Cross-Regional Horizontal and Vertical Transport Pathways of Carbon Monoxide and Its Impact on Air Pollution in the Tibetan Plateau

Abstract

Carbon monoxide (CO) is an ideal tracer for tracking cross-regional transport of anthropogenic pollution due to its longer atmospheric lifetime. This study analyzed the seasonal variations of CO over the Tibetan Plateau (TP) using surface and satellite observations, focusing on the impact of transport processes on the three-dimensional distribution of CO across the TP and surrounding regions. Unlike regions with significant surface emissions, the TP exhibited opposing seasonal patterns and a negative correlation between surface CO and CO total columns (COTC). Surface CO peaked in winter/spring, while COTC peaked in summer, due to different contributions of transport processes at various altitudes. CO from tropical biomass burning was transported up and north by the Hadley circulation, increasing CO concentrations with altitude over the TP, except for winter, when CO stayed uniform within the troposphere, indicating seasonal variations in transport. During winter/spring, downdrafts transported high CO concentrations to the lower troposphere, where it was efficiently removed by strong winter westerlies. In summer/autumn, strong convection lifted surface CO from India and Southeast Asia into the upper troposphere over the TP. These findings indicated that high COTC over the TP and other regions influenced by similar circulations should not be misinterpreted as surface air pollution. Furthermore, using low CO as an indicator of stratospheric intrusion as in current studies might be unsuitable for tropical/subtropical regions where CO was elevated near the tropopause. Future research should further explore these transport mechanisms and their broader implications for atmospheric chemistry and climate dynamics over the TP.